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The above checks out if depreciation is 10% for example: 100 in 2000, 90 in 2001, 81 in 2002 and 72.9 in 2003.

Challenge answer is sqrt S*T
How?

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12. degrees between 12:24 and 14:36 hour hand.

Can someone please explain the solution to me? I don't understand. I've seen this kind of solution before and not understanding worries me. I got it right though by doing the following and am not sure if the method is right.

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18B. 3 times as many pupils in 2nd as in 1st. Solution says y=3x therefore x cancels out so result can be calculated. If x cancels out, what is the answer?

I got (12.2x+13.1y)/x+y; or 12.2x+13.1*3x/x+3x; = 51.5x/4x;
then??
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19. The given equation gets simplified to x+y=10. And this encloses a coordinate plane.
solution: 10^2+10^2= sqrt of 200
BUT what is the assumption in arriving at this solution.

I was at a loss so decided to do (x+y) (x+y) = 10 * 10 = 100 as area, though wasn't sure why I was doing that either! (and it appears to be incorrect anyway)